1. Field of the Invention
The invention relates to a method for filling gaps in integrated circuits with low dielectric constant material by varying the rf bias applied to a substrate in a high density plasma chemical vapor deposition process.
2. Statement of the Problem
As semiconductor technology advances, circuit elements and interconnections on wafers or silicon substrates become increasingly more dense. As a result of the continuing trend toward higher device densities, parasitic interdevice currents are increasingly problematic. In order to prevent unwanted interactions between these circuit elements, insulator-filled gaps, or trenches, located between active circuit devices and metallized interconnect layers are provided to physically and electrically isolate the elements and conductive lines. However, as circuit densities continue to increase, the widths of these gaps decrease, thereby increasing gap aspect ratios, typically defined as the gap depth divided by the gap width. As the gaps become narrower, parasitic capacitance increases, and filling the gaps with insulating material becomes more difficult, and this can lead to unwanted voids and discontinuities in the insulating, or gapfill, material.
For example, in metal-oxide-semiconductor (xe2x80x9cMOSxe2x80x9d) technology, it is necessary to provide an isolation structure that prevents parasitic channel formation between adjacent devices, such devices being primarily NMOS and PMOS transistors or CMOS circuits. Trench isolation technology has been developed in part to satisfy such insulation needs. Refilled trench structures essentially comprise a recess formed in the silicon substrate that is refilled with a dielectric insulating material. Such structures are fabricated by first forming submicron-sized trenches in the silicon substrate, usually by a dry anisotropic etching process. The resulting trenches typically display a steep side-wall profile. The trenches are subsequently refilled with a dielectric, such as silicon dioxide, typically by a chemical vapor deposition (xe2x80x9cCVDxe2x80x9d) technique. They are then planarized by an etchback process so that the dielectric remains only in the gap, its top surface level with that of the silicon substrate. The resulting filled-trench structure functions as a device isolator having excellent planarity and potentially high aspect ratio beneficial for device isolation. Refilled trench isolation can take a variety of forms depending upon the specific application; they are generally categorized in terms of the trench dimensions: shallow trenches ( less than 1 xcexcm), moderate depth trenches (1 xcexcm to 3 xcexcm), and deep, narrow trenches ( greater than 3 xcexctm deep,  less than 2 xcexcm wide). Shallow Trench Isolation (STI) is used primarily for isolating devices of the same type in increasingly dense MOS circuits. STI provides a high degree of surface planarity.
Similar isolation techniques are used to separate closely spaced circuit elements that have been formed on or above a semiconductor substrate during integrated circuit fabrication. The circuit elements may be active devices or conductors, and are isolated from each other by refilled xe2x80x9cgapsxe2x80x9d.
The basic trench, or gap, isolation process is, however, subject to drawbacks, one of these being void formation in the gap during dielectric gap fill. Such voids are formed when the gap-filling dielectric material forms a constriction near the top of the gap, preventing deposition of the material into the gap interior. Such voids compromise device isolation, as well as the overall structural integrity. Unfortunately, preventing void formation during gap fill often places minimum size constraints on the gaps themselves, which may compromise device packing density or device isolation.
Silicon dioxide is formed by conventional CVD techniques by mixing a gaseous oxidizer (e.g., N2O), silane (SiH4) and inert gases, such as argon, and energizing the mixture in a reactor so that the oxygen and silane react to form silicon dioxide on a wafer substrate. Currently, plasma-enhanced chemical vapor deposition (xe2x80x9cPECVDxe2x80x9d) processes are used to fill gaps with silicon oxide material. In PECVD processes, a plasma of ionized gas is formed in the CVD plasma reactor. The plasma energizes the reactants, enabling formation of the desired silicon dioxide at lower temperatures than would be possible by adding only heat to the reactor system. In a typical plasma-enhanced CVD (xe2x80x9cPECVDxe2x80x9d) process, the plasma is a low pressure reactant gas discharge that is developed in a radio-frequency (xe2x80x9crfxe2x80x9d) field. The plasma is an electrically neutral ionized gas in which there are equal number densities of electrons and ions. At the relatively low pressures used in PECVD, the electron energies can be quite high relative to heavy particle energies. The high electron energy increases the density of dissociated reactants within the plasma available for reaction and deposition at the substrate surface. The enhanced supply of reactive free radicals in the PECVD reactor enables the deposition of dense, good quality films at lower temperatures (e.g., 400xc2x0 C.) and at faster deposition rates (30 nm/min to 40 nm/min) than typically achieved using only thermally-activated CVD processes (10 nm/min to 20 nm/min).
In addition to silane (SiH4), other silicon-containing precursors have been used to form silicon dioxide, including disilane (S2H6) and tetraethoxysilane (xe2x80x9cTEOSxe2x80x9d). All of these processes require mixing the silicon-containing reactant with an oxidizing gas reactant, such as oxygen gas (O2), ozone (O3), nitrous oxide (N2O), nitrogen dioxide (NO2) or carbon dioxide (CO2).
The capacitance across a gap is governed by the formula
C=xcex5kA/t
where C is the capacitance, xcex5 is the dielectric constant of the gap fill material, k is a constant, A is the area of the gap (i.e., the area of the side of the circuit element forming the gap), and t is the thickness or width of the gap. As gap widths decrease with increasing density, the capacitance across the dielectric gap fill material increases. Thus, as integrated circuits become increasingly dense, decreasing t, it is necessary to lower the dielectric constant of the gap fill material to reduce cross-talk, capacitive coupling and resulting speed degradation, and power consumption. To compensate for smaller gap dimensions, it is known to substitute dielectric materials having dielectric constants lower than silicon dioxide. It is known in the prior art to form halogen-doped silicon dioxide. For example, fluorinated silicon dioxide films possess a dielectric constant of approximately 3.2, whereas typical CVD-SiO2 has a dielectric constant of about 3.9. It is also known to use multilayer or xe2x80x9csandwichxe2x80x9d gap-fill material, including structures comprising a silicon dioxide layer and a non-silicon carbon-based layer or a polymer layer. Although these structures may possess an overall dielectric constant lower than silicon dioxide, their fabrication processes are slow, complex and expensive, and they are limited with respect to aspect ratios achieved.
Design feature widths of integrated circuit devices are currently approaching 0.25 xcexcm, or 250 nm. To achieve corresponding overall circuit density, gap dimensions of approximately 100 nm to 400 nm gap width range and 300 nm to 1000 nm gap depth range are desired, having corresponding range of aspect ratios of 2 to 6. Furthermore, because the gap is so thin, the insulating gap material should have a dielectric constant of 3.3 or less.
A gap opening of 500 nm or less is too small for depositing material using conventional CVD and PECVD methods. Also, as the deposition of gap-filling material proceeds, the gap opening becomes smaller, making it more difficult to fill and creating the risk of void formation. Currently, high density plasma (xe2x80x9cHDPxe2x80x9d) CVD is used to fill high aspect ratio gaps. Also, using HDP-CVD, it is usually possible to deposit silicon oxide films at lower temperatures (e.g., 150xc2x0 C. to 250xc2x0 C.) than in a PECVD process. Typical HDP-CVD processes use a gas mixture containing oxygen, silane, and inert gases, such as argon, to achieve simultaneous dielectric deposition and etching. In an HDP process, an rf bias is applied to a wafer substrate in a reaction chamber. Some of the gas molecules, particularly argon, are ionized in the plasma and accelerate toward the wafer surface when the rf bias is applied to the substrate. Material is thereby sputtered when the ions strike the surface.
It is known that carbon-containing silicon oxide films have lower dielectric constants than silicon oxide films. It is believed that the carbon works by decreasing the effective density of the film, since a film of zero density, that is, a vacuum, has a dielectric constant of 1. Also, carbon contained in silicon oxide films, for example, CH3 groups, is usually less polarizable than silicon oxide, thereby lowering the capacitance, or dielectric constant, of the thin film.
It is known in the art to deposit a carbon-containing silicon oxide film by reacting an organic carbon-containing gaseous precursor compound, a silicon-containing gaseous precursor compound and an oxygen-containing oxidizing reactant gas (xe2x80x9coxidizerxe2x80x9d) in a PECVD or HDP-CVD process. The oxidizer serves to oxidize silicon atoms to form silicon oxides. The carbon-containing and the silicon-containing precursors may be the same organic precursor compound. The oxygen containing reactant gas may be oxygen gas (O2), ozone (O3), nitrous oxide (N2O), nitrogen dioxide (NO2), carbon dioxide (CO2) or other oxidizer gas. During formation of the insulator film in conventional methods for forming silicon oxide films containing carbon, the reactive oxygen from the oxygen-containing reactant gas (xe2x80x9coxidizerxe2x80x9d) oxidizes both silicon atoms and carbon atoms. The reactive oxygen oxidizes the carbon present in both the deposited thin film and in the gaseous precursor. The products of the oxidation reaction include CO and CO2, which are volatile gases that escape from the film. As a result, the deposited gap-fill film contains less carbon and has a higher dielectric constant than desired. Use of oxidizer gases less reactive than O2 gas ameliorates the negative effects of carbon oxidation; yet, the problem of undesired oxidation of carbon persists.
When a carbon-containing silicon-oxide is deposited to fill narrow gaps having aspect ratios of about 1.5 or greater, the simultaneous deposition and sputtering of an HDP-CVD process is necessary in order to fill the gap. Unfortunately, however, the high frequency sputtering preferentially removes the carbon groups, such as CH3, from the deposited thin film material. The reduction in carbon-content of the thin film results in increased dielectric constant, which is undesired.
Thus, there is a need for a method of depositing gap-filling insulator material for filling an insulator gap, or a trench, having an aspect ratio of 2 or greater and containing insulating material having a dielectric constant of 3.3 or less. In particular, there is a need for a method of depositing a carbon-containing silicon oxide thin film in a narrow gap that does not reduce the carbon-content of the thin film.
The invention provides a novel method for filling insulator gaps, or trenches, with low dielectric constant material. A method in accordance with the invention includes using a high-density plasma CVD (xe2x80x9cHDP-CVDxe2x80x9d) technique to deposit dielectric insulator material in the gaps to be filled. A key, novel feature of the invention is varying the high frequency (xe2x80x9chigh Fxe2x80x9d) rf bias applied to a wafer substrate during the process of filling a gap with dielectric insulator material. The rf bias may be varied by applying a pulsed rf bias or by tailoring the magnitude of the rf bias, or both. In contrast to the invention, a continuous, uniform rf bias is applied to the wafer substrate in conventional HDP-CVD processes.
To apply a pulsed bias in accordance with the invention, preferably, a bias trigger is added to the high F rf source, which bias trigger variably controls the period and length of on-time of the rf source. The bias trigger switches the high F rf bias on and off for various amounts of time. The amounts of time on and off can be constant throughout the deposition process, or they can vary throughout the deposition process. The pulse timing may be pre-set at the start of the deposition process, or it may be controlled through a monitoring and feed-back process-control system. A tailored rf bias in accordance with the invention is a high F rf bias of which the magnitude of the bias is tailored to change during the deposition process. Typically, the magnitude decreases during the deposition process so that it is the minimum power necessary to preferentially sputter-etch the deposited gap-filling dielectric material at the top of the gap to prevent the gap opening from prematurely closing and forming a void in the gap. The magnitude is typically tailored by a setting on the bias generator. In accordance with the invention, the rf bias may be varied by applying a pulsed bias, or a tailored bias, or a pulsed tailored bias.
When an HDP-CVD method in accordance with the invention is used, the high F rf bias applied to the substrate results in sputter etching of the deposited thin film by highly energized radicals and inert gas molecules of the plasma. As a result, dielectric material deposited on the wafer surface is sputter-etched, helping to keep gaps open during the deposition process. By varying the bias to minimize sputter-etching, the overall rate of deposition is increased, operating and equipment costs are lowered, and damage to the circuit elements and to the dielectric thin film is minimized. A method in accordance with the invention is especially useful for deposition of a carbon-containing silicon oxide gapfilling dielectric material. A method employing HDP-CVD is especially useful to fill isolator gaps having a gap width in the range of from 100 nm to 1000 nm and an aspect ratio in the range of from 2 to 3 or greater.
Preferably, the variable high F rf is applied to the substrate as the energized plasma is reacting and solid oxide material is being deposited on the substrate, so that sputtering and deposition occur simultaneously. Deposition and sputtering may also be conducted in sequence, however.
The reactant gas mixture in an HDP-CVD reaction chamber in accordance with the invention typically comprises a silicon-containing precursor compound and an inert plasma forming gas, such as argon. Argon is also typically used as the sputtering gas. The reactant gas mixture typically comprises an inert carrier gas that carries the organic precursor compound into the reaction chamber. Preferably the organic precursor compound contains silicon, oxygen and carbon. The reactant gas mixture also may comprise organic solvent molecules.
A method in accordance with the invention is especially useful for filling gaps with carbon-containing silicon oxide dielectric material. The carbon atoms in carbon-containing silicon oxide material are typically present in alkyl groups, such as CH3-groups. The carbon-containing groups are more easily sputtered from the surface of the deposited dielectric material than silicon oxide. Thus, the carbon is preferentially removed from the dielectric material, reducing the carbon content of the carbon-containing dielectric material. As a result of varying the high F rf bias in accordance with the invention to minimize the time or the magnitude of the sputter-etching, or both, less carbon is removed from the deposited dielectric material.
In a preferred embodiment of the invention, an organic precursor compound containing silicon, carbon and oxygen reacts in a plasma-enhanced CVD process without the addition of an additional, oxygen-containing reactant gas to form a carbon-containing silicon oxide layer. In contrast with other embodiments, which add a separate oxygen-containing oxidizer gas to the reactor chamber to oxidize silicon, the preferred embodiment of a method in accordance with the invention does not comprise the addition of O2 gas or N2O or other oxidizer gases to the reaction chamber or to the reactant gas mixture of the CVD process. Instead, the oxygen for forming the silicon oxide of the insulator film is provided in the organic precursor of the invention. Since the oxygen in the precursor is chemically bound with the silicon, it is relatively unreactive, compared with oxygen atoms of O2, ozone, N2O and other oxidizer gases commonly used in CVD processes. As a result, a preferred embodiment of a method in accordance with the invention avoids oxidation of the carbon in the precursor and in the deposited thin film. The carbon in the thin film is chemically stable, typically in the form of carbon-containing groups, such as xe2x80x94CH3. Since there is no reactive oxidizing gas added to the CVD reaction chamber, the only reacting oxygen comes from the precursor. The plasma oxygen concentration is lower, so volatile carbon compounds, such as CO2 or CO, do not form and volatilize as much as if a reactive oxidizing gas were present. Also, since the oxygen atoms are bound, they do not react with hydrogen atoms that may be present, forming undesired OH-groups or water, which would increase the dielectric constant. A method in accordance with the invention, including varying the high F rf bias applied to the wafer substrate in an HDP-CVD reaction chamber, is especially useful to fill isolator gaps having a gap width in the range of from 100 nm to 1000 nm and an aspect ratio in the range of from 2 to 3 or greater with carbon-containing silicon oxide material having a low dielectric constant, typically less than 3.5.
Preferably, the gaseous precursor stream flowed into the CVD reactor contains an organic precursor compound comprising silicon, oxygen and carbon. In a method in accordance with the invention, the gaseous precursor stream may contain a plurality of precursor compounds and each precursor compound may contain one or several of the elements silicon, oxygen and carbon. An important feature of a method and of organic precursors in accordance with the invention, however, is that the oxygen atoms contained in the gaseous precursor are bound in a relatively unreactive form. Typically, the oxygen atoms are chemically bound to silicon in a precursor compound. A preferred precursor is octamethylcyclotetrasiloxane.
The invention, therefore, is most useful when applied to deposit a thin film of low dielectric constant, carbon-containing silicon oxide in a gap, or trench, between active devices or conductive interconnects in high density integrated circuits. A method using a pulsed bias or tailored bias in accordance with the invention is also useful, however, to deposit silicon dioxide films having conventional compositions, especially when filling gaps having extremely high aspect ratios (e.g., when the aspect ratio is up to 4). In addition to filling gaps or trenches, however, the invention may be used to deposit a premetal dielectric layer (xe2x80x9cPMDxe2x80x9d), an intermetal dielectric layer (xe2x80x9cIMDxe2x80x9d), an interlayer dielectric layer (xe2x80x9cILDxe2x80x9d), a passivation layer and other insulator thin films having a low dielectric constant in an integrated circuit.